1. Field of the Invention
The present invention relates to the temporary storage of products and particularly to a method of palletizing and depalletizing stacks of substantially flat and thin articles in continuous, layer-by-layer succession. More specifically, this invention is directed to the storage and recovery, during the assembly of printed publications, of folded sheets of paper and the like and especially to apparatus which permits such flat and thin products to be stored and retrieved from storage layer-by-layer and in continuous fashion. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
In the manufacture of books, brochures, periodicals and similar products, it is often necessary to stack large quantities of printing works products on pallets for storage pending further processing and to subsequently retrieve such products from storage. Methods and apparatus have previously been proposed which enable such printing works products, i.e., portions of printed publications, to be stacked on pallets. In the previous methods and apparatus, the unloading of the individual stacks of printing works products has commonly been mostly performed by hand. This hand unloading is an extraordinarily labor intensive endeavor, and thus costly, and also demands heavy physical effort. Furthermore, a persistent problem results from the fact that the individual printing works products that are brought together to form stacks cannot be prevented from shifting relative to one another, especially during hand unloading.
As an example of the prior art, in order to load a book assembling machine, a stacker truck is utilized for transporting the stacks of folded printed sheets, which have been temporarily stored on pallets, from the in-process storage area to the feeding or loading apparatus of the assembling machine. When the pallet bearing the stacks of folded printed sheets reaches the assembling machine, an operator must unload the pallet and feed the individual stacks of printed sheets to the assembling machine by hand. This is an extremely difficult and tiring job and it is virtually impossible to perform the job without the shifting, and subsequent disordering, of the printing works products.
For an example of prior art apparatus for loading printing works products onto pallets in layers, reference may be had to published German Patent Application 31 07 495. This published application discloses a system for depositing stacks of printed newspaper or book pages, or other similar printed matter, on a pallet so that the thin flat products in question are aligned in a specific direction. The apparatus of the published application operates on the push-off principle and includes a sliding platform onto which an assembled group of the products is transferred by means of a pusher element. The sliding platform, after being loaded, moves to a position above the pallet and, after a push-off element has been lowered behind the group of products, the sliding platform is lowered to a level which is adjacent a layer of products already present and then retracted beneath the push-off elements. The end result is that the group of products is swept off onto the pallet or onto a layer of products which has already been deposited on the pallet during platform retraction.
In stacking appliances of the type generally described above which utilize a sliding platform and a cooperating push-off element, the overall height of the platform structure results in a relatively large difference in height between the supporting surface of the platform and the top of the pallet or the top of the layer of products already present on the pallet. Due to this height difference, the products being stacked drop through a relatively large distance when being swept off the sliding platform. This relatively long free fall unavoidably leads to the products, which have been precisely aligned on the sliding platform in a predetermined collective arrangement conforming to a defined stacking order, losing the predetermined collective arrangement. This disordering is most prevalent when the products, i.e., the book pages or the like, are thin. That is, there is particular risk that very thin printing works products will slide over one another with the result that an unstable stack will be formed. Since it is seldom possible to perform an aligning operation once stacking has been completed, serious alignment problems are encountered during destacking.
Continuing with the above discussion, as a result of the friction that occurs between the products and the supporting surface as the products are being pushed sideways onto the sliding platform of the prior art stacking appliances, the same friction being encountered when the products are pushed off the platform, prior art palletizing installations are incapable of meeting the requirement that products which are highly susceptible to damage, such as stacked portions of books or brochures, be treated with exceptional gentleness.